Direct dental filling materials consisting of a resin binder and a finely divided inorganic filler are well known to those skilled in the art. Since the disclosure of such materials in Bowen U.S. Pat. No. 3,066,112, variations and improvements thereon have been disclosed in U.S. Pat. Nos. 3,179,623; 3,194,783; 3,194,784; 3,835,090) and 3,926,906.
The dental filling materials to which the present invention pertains employ a resin binder system comprising a polyfunctional monomer having at least two acrylic end groups, as exemplified by 2,2-bis[4-(3-methacryloxy-2-hydroxy-propoxy)-phenyl]-propane (BIS-GMA) and other monomeric materials, as taught in the above-identified references, the disclosures of which are hereby incorporated by reference. Also preferably included in the resin system are other active monomers referred to by Bowen as reactive diluents, the function of which is to reduce the viscosity of the resin binder system.
For use as dental filling materials, the resin binder system, e.g., a mixture of BIS-GMA and a reactive diluent, is mixed with a large proportion, generally 65% by weight or more, of a finely divided inorganic filler material having a particle size within the range of about 1-85 microns, such as fused silica, crystalline quartz, aluminum oxide, and glass beads. There is also added to the mixture of the resin binder and the inorganic filler, an appropriate quantity of a free radical generating catalyst, such as benzoyl peroxide, and a suitable activator such as N,N-di-methyl-p-toluidine. The free radicals which are generated by the combination of the catalyst and the activator lead to rapid polymerization of the resin binder system, producing a dental filling material which has a desirable combination of properties, including high stiffness, high compressive strength, low shrinkage on hardening, and a low coefficient of thermal expansion, as known by those skilled in the art.
In the preparation of such dental filling materials, it is desirable to incorporate as high a proportion of inorganic filler as possible, in order to maximize the strength, rigidity and wear properties of the cured product. As a practical matter, however, it has been found that the use of more than about 75% of inorganic filler presents increasing difficulty, since the resulting paste material becomes non-homogenous and/or too stiff to be placed properly in a dental cavity and shaped to the desired contours, in accordance with the usual procedures employed with such dental filling materials.
It is also known in the prior art to pretreat the inorganic filler material with an ethylenically unsaturated organosilane finishing or keying agent, such as tris(2-methoxyethoxy) vinylsilane. Such treatment improves the bond between the organic polymer binder and the surface of the finely divided filler particles, thereby increasing the strength of the cured dental filling material, as well as making the filler desirably hydrophobic.
In addition to such fillers, used to impart the desired mechanical properties to the cured composite, it has also been suggested in U.S. Pat. No. 3,926,906 to incorporate a small proportion, typically 1-4% by weight of the total paste mixture, of sub-micron silica in order to prevent separation of the liquid phase of a paste mixture which might occur on storage